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Definition

When excessive hemoglobin in the blood is converted to another chemical that cannot deliver oxygen to tissues, called methemoglobin.

Description

The molecule hemoglobin in the blood is responsible for binding oxygen to give to the body. When hemoglobin is oxidized to methemoglobin its structure changes and it is no longer able to bind oxygen. Hemoglobin is constantly under oxidizing stresses: however, normally less than 1% of a person's hemoglobin is in the methemoglobin state. This is due to the body's systems that reduce methemoglobin back to hemoglobin. Infants have a higher risk of acquiring methemoglobinemia because infant hemoglobin is more prone to be oxidized to methemoglobin.

Causes and symptoms

Methemoglobinemia can either be congenital or acquired.

There are two causes of the congenital form. One cause is a defect in the body's systems to reduce methemoglobin to hemoglobin. The other cause is a mutant form of hemoglobin called hemoglobin M that cannot bind to oxygen. Both of these forms are typically benign.

Acquired methemoglobinemia is caused by an external source, usually a drug or medication. Some of these medications include benzocaine, lidocaine and prilocaine. These medications can inhibit the body's systems of reducing methemoglobin to hemoglobin resulting in methemoglobinemia.

With a methemoglobin level of 3-15% skin can turn to a pale gray or blue (cyanosis). With levels above 25% the following symptoms may be present:

• Cyanosis unaffected by oxygen administration
• Blood that is dark or chocolate in color that will not change to red in the presence of oxygen
• Headache
• Weakness
• Confusion
• Chest pain

When methemoglobin levels are above 70% death may result if not treated immediately.

Diagnosis

Diagnosis is based on the symptoms and history. If these are indicative of methemoglobinemia blood tests are performed to confirm the presence and level of methemoglobin.

Treatment

For acquired methemoglobinemia the typical treatment is with methylene blue. This is administered with an IV over a five-minute period and results are typically seen within 20 minutes. Methylene blue reduces methemoglobin back to hemoglobin.

Though congenital methemoglobinemia is usually benign, the form due to a defective reducing system can be treated with ascorbic acid (vitamin C) taken daily. The other congenital form due to hemoglobin M has no treatment as of late.

Alternative treatment

There are not any known alternative treatments for methemoglobinemia. Methylene blue, or a similar treatment, is needed to reduce methemoglobin to hemoglobin.

Prognosis

If found early, acquired methemoglobinemia can be easily treated with no side effects. After treatment with methylene blue the patient can expect a full recovery.

Congenital methemoglobinemia is typically benign and should be observed. If methemoglobinemia symptoms occur the person should be taken to the hospital for treatment.

Prevention

If a person gets methemoglobinemia from a certain medication that medication should be avoided at all costs in the future. For people with congenital methemoglobinemia medications or other things that are known to oxidize hemoglobin should be avoided.

Key Terms

Cyanosis

When the body does not receive enough oxygen.

Oxidation

When a chemical element or compound loses an electron.

Reduction

When a chemical element or compound gains an electron.

For Your Information

Resources

Books

• Beutler, Ernest. "Methemoglobinemia and Other Causes of Cyanois." In Williams Hematology. 6th ed. New York: McGraw-Hill, 2001, pp. 611-17.

• Wilburn-Goo, Dawn, and Lloyd. "When Patients Become Cyanotic: Acquired Methemoglobinemia." Journal of the American Dental Association June 1999: 826-31.
• Wright, Lewander, and Woolf. "Methemoglobinemia: Etiology, Pharmacology, and Clinical Management." Annals of Emergency Medicine November 1999: 646-56.

• eMedicine. Website. 2001.

This article was updated on 08-14-2006